speedbump6 is right. The "variety of reasons" is really one: bass modes. Because, physics, every sub in every room has areas where bass frequencies reinforce or cancel out. The old school approach was to move the sub around trying to find the smoothest response, then maybe try and EQ out the inevitable highs and lows.
EQ never really works. The problem is that while EQ can get even response at one location, the resulting uneven signal is being put out by the sub all the time. So in all the regions being boosted too much bass is being put into the room. All the sonic energy that goes into the room from a speaker bounces around the room exciting everything in the room and causing it to reverberate and emit its own delayed sound right back into the room. By boosting the bass EQ actually makes this worse. The poor person using EQ then has to run out and spend even more money on tube traps trying to suck up all the extra bass he paid all that money to put into the room in the first place. So many audiophiles still think this is the way to go. Sigh.
DBA takes advantage of room modes by putting several subs in different locations. All the different locations create bass modes in different areas. Because there are so many they add together so no single one has to be all that powerful. Because they are less powerful the lumps in their bass response is less as well. All together the result is much, much smoother bass. Without EQ. Without all the extra energy going into the room the bass is fast, taut, articulate. Like you never heard before.
Its one thing to talk about. Its quite another to actually experience. Its not that the other ways of doing this don't work at all. They do. Just not anywhere near as good.
Its hard to have too much bass. Four or five Brisance might just get you there. I would go with four of the 4-10's and one or two Dayton SA-1000 sub amps. Or if you already have some subs then whatever it takes to get you to four or five. Then you can either tweak what tiny amount of uneven response remains with your EQ, or ditch it altogether.
